In recent years, an attempt to desalinate seawater has been made in the coastal big cities in arid or semi-arid areas where it is difficult to ensure water resources in a stable manner. Further, in areas poor in water resources such as China and Singapore, an attempt to purify industrial wastewater and domestic wastewater has been made for reusing the water. More recently, reuse of wastewater discharged from oilfield plants or the like has also been attempted by removing oils and salts from the oil-contaminated wastewater having high turbidity. A membrane method using a composite semi-permeable membrane has been found to be effective for such water treatment in terms of cost and efficiency. In such a water treatment method, water to be treated is continuously supplied to a membrane module having a spiral-type composite semi-permeable membrane element at a high pressure of about 1 to 7 MPa (see Patent Document 1 or 2).